1. Field of the Invention
The present invention relates to a zoom lens preferably used in a digital camera configured to obtain subject image data via an image pickup device such as a solid-state image pickup device, more particularly, to a zoom lens preferably used as an imaging optical system in a video camera for obtaining a moving image as well as a still image camera for obtaining a still image and also in a silver salt camera using a silver salt film as an image recording medium, and also to an information device having the zoom lens, an imaging apparatus having the zoom lens as an imaging optical system, and the like.
2. Description of the Related Art
Recently, instead of a still camera in a film type using a silver salt film as an image recording medium, that is, a silver-salt camera, an imaging apparatus such as a digital camera obtaining a still camera, a video or a moving image of a subject via a solid-state image pickup device such as a CCD (charge-coupled device) image pickup device, a CMOS (complementary metal-oxide semiconductor) image pickup device, and the like and an information device having such a camera function, for example, a personal digital assistant, have been widely used. User requirements in the above devices have been diversified and particularly, of the user requirements, an imaging apparatus having a small size and high variable power ratio at a wide field angle has been required.
In order to downsize a zoom lens used as an imaging optical system in such an imaging apparatus, it is required to reduce an entire length of lenses when changing magnification of the zoom lens (that is, a distance from a lens surface at the most object side to an image plane), more particularly, to reduce an entire length of the lenses at a telephoto end. Furthermore, in order to downsize the imaging apparatus, it is important to suppress the entire length of the lenses at a collapsed state by reducing a thickness of each lens group. In order to achieve high performance of the zoom lens, considering possibility of application to the high-end digital camera, resolution corresponding to an image pickup device having at least more than 5 to 10 million pixels is required over an entire zoom range.
In order to achieve a wide filed angle of the imaging optical system, it is preferable to achieve a half field angle of 38 degree or more at a wide angle end. The half field angle of 38 degrees corresponds to a focal length of 28 mm in a case of a silver-salt camera using a silver-salt film (so-called Leica film) having a width of 35 mm.
In order to achieve a high variable power ratio, a zoom lens having an ability of changing a magnification from a focal length of 28 mm to 300 mm, that is, about ten times of the variable power ratio is sufficient for a general photographing.
Although there are various types of the zoom lens for a digital camera, the zoom lens having five or more lens groups is not suitable for a small size camera because it is difficult to reduce a total thickness of the lens groups.
As the zoom lens having four lens groups suitable for a high variable power ratio and large aperture, an example of a zoom lens including a first lens group having a positive refracting power, a second lens group having a negative refracting power, an aperture stop, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power, which are disposed in order from an object side, when changing the magnification of the zoom lens from a wide angle end to a telephoto end, an interval between the first and second lens groups is increased, an interval between the second and third lens groups is decreased, an interval between the third and fourth lens groups is changed, and the second lens group has a negative lens, a positive lens, and a negative lens, in order from the object side is disclosed in Japanese Patent Application Publication Nos. 2004-199000, 2005-326743, 2008-76493, 2008-96924, 2008-26837, 2008-112013, and 2008-107559.
The zoom lens disclosed in Japanese Patent Application Publication Nos. 2004-199000, 2005-326743, 2008-76493, and 2008-96924 has a variable power ratio of about 2.8 to 6.8 times and therefore it is not sufficient for satisfying the user's requirements.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-26837 has a variable power ratio of about 10 times but has a large telephoto ratio and therefore is not preferable for providing a compact size.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-112013 has a remarkably large variable power ratio, that is, about 9.5 times and a field angle of about 40 degrees at the wide angle end as well as a relatively small telephoto ratio. In this point, user requirements are satisfied, but lens performance, aberration such as color aberration can be further improved. That is, referring to the specific configurations shown in Japanese Patent Application Publication No. 2008-112013, as described above, a wide half field angle and a sufficient variable power ratio can be obtained at the wide angle end. However, at the telephoto end, distortion is large, that is, 10% or more and therefore, even when the distortion is corrected, the corrected amount is large so that an image is degraded.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-107559 has, in a specific configuration example, a wide half field angle, that is, about 40 degrees at the wide angle end, but the variable power ratio is about 5 times. In addition, the distortion is about 10% at the wide angle end and therefore even when the distortion is corrected, the corrected amount is large so that the image is degraded.
As described above, the zoom lens disclosed in the above-described prior arts does not sufficiently satisfy the user requirements, that is, high variable power ratio, wide field angle and small size are achieved while the aberration is reduced. It is required to provide a zoom lens having high variable power ratio, wide field angle, and small size as well as reduced aberration.